1. Field of the Invention
The present invention relates in general to a wired base plate and package for electronic parts, and more particularly to a LGA (Land Grid Array) type wired base plate for mounting thereon an integrated circuit chip and an electronic part package having such an LGA type wired base plate on which an integrated circuit chip is mounted and which is hermetically sealed by means of a lid.
2. Description of the Related Art
Heretofore, a LGA type wired base plate for electronic parts (hereinafter referred to as LGA type wired base plate or simply as wired base plate) has been formed from high strength ceramic such as alumina by firing simultaneously with internal conductor wire layers and vias which are disposed inside the wired base plate. FIG. 8 shows an example of a prior art wired base plate 101. As shown, an electronic part 121 including an integrated circuit chip, transistor, capacitor and the like (hereinafter referred to simply as integrated circuit chip) is installed on the wired base plate 101 and electrically connected to same. A lid or cap 122 is bonded to the wired base plate 101 by soldering or the like, whereby an electronic part package 131 is completed.
In the meantime, such an electronic part package 131 is generally installed or mounted on a circuit board (exterior circuit board) in the following manner. For example, as shown in FIG. 9, a number of pads (connecting terminals) 105 formed on a lower or rear surface of the wired base plate 101 are brought into contact with respective pads 143 of a circuit board 142 by way of an intermediate connecting member (hereinafter referred to as a spring connector) 141 having a springy property, i.e., a resilience like a spring, and in such a state screw members 145 extending through a pushing plate 144 which is placed on the lid 122 and through the circuit board 142, are screwed into rear plates (metallic plate) 146 attached to a rear surface of the circuit board 142 and tightened with a predetermined torque to join them together. By such installation or mounting, the number of pads 105 on the rear surface of the wired base plate 101 and the number of pads 143 of the circuit board 142 are always brought into contact with each other under spring pressure to assure the electrical connection therebetween.
In the meantime, the spring connector 141 shown in FIG. 9 has an insulator in the form of a plate and made of rubber or flexible resinous material and a number of metal spring pieces having a bent shape (i.e. , like &lt;). The bent metal spring pieces are disposed vertically and horizontally of the spring connector 141, i.e., at corresponding positions to the pads 105 and 143 so as to be resiliently deformable or compressed by the above pressure and brought into contact with the pads 105 and 143 under spring pressure. As will be understood from such a structure, in order to retain electrical connections between such a number of pairs of pads, it is indispensable for the both pads, i.e., a number of pairs of pads are pressed against each other with a considerable load. For example, a load necessary for such an end (hereinafter referred to simply as pressure or pushing pressure) is generally 50 to 100 g/pad (one contact point). Accordingly, in case of a wired base having 1000 pads, the wired base is always subjected to a load of from 50 to 100 Kg.
On the other hand, as the integrated circuit chip has come to operate at high speed, there may occur such a case in which a conventional wired base plate made of alumina ceramic cannot satisfy the requirements for its electrical characteristics such as impedance, capacitance, crosstalk and so on. Thus, to meet with such requirements for the high speed of the integrated circuit chip, a trial has been made to use such a wired base plate 201 shown in FIG. 10., i.e., a wired base plate including a substrate (single plate or laminated plate) 202 of ceramic such as alumina as a base, and a plural wire layer portion (multi-wire layer portion) 203 disposed on the substrate 202. The plural wire layer portion 203 includes a plurality of insulation layers of polyimide resin (hereinafter referred to simply as polyimide) or the like resin having a low dielectric constant as compared with alumina and a plurality of conductor wire layers which are laminated or placed alternately on upon another.
However, polyimide or the like resin is weak in the mechanical strength (compression) as compared with ceramic and furthermore considerably small in Young's modulus so it is easily deformed by compression. Thus, the LGA type wired base plate 201 formed in the above manner is subjected to a large pressure in case of installation or mounting on a circuit board as described above, there are caused the following problems. As shown in FIG. 11, an integrated circuit element 121 is mounted on such a wired base plate 201 and then a lid 122 is bonded or attached to the base plate 201 to complete a package. Thereafter, in mounting of the package on a circuit board, a pressure is applied to the package through the lid 122. When this is the case, all of the pressure is applied through the lid 122 to an outer peripheral part (i.e., lid bonding surface) of the plural wire layer portion 203.
Due to this, as shown in FIG. 11, the polyimide is caused to deform largely by the pressure P at or adjacent the surface area to which the lid 122 is bonded (i.e., the surface area indicated by the width W in the figure), so that a lid bonding section of the plural wire layer portion 203 is caused to sink or depressed relatively from an inner section of same by a deformation amount S. Such sinking or depressing, i.e., such deformation S by compression means that the plural wire layer portion 203 is always subjected to a shearing force at the border (i.e., the portion along the line K--K in FIG. 11) between the region (i.e., the region indicated by the width W) to which the lid 122 is bonded and the region to which the lid 122 is not bonded. Accordingly, the wired base plate 201 encounters a problem that there is a possibility of breakage or disconnection of the conductor wire layer 206 disposed inside the base plate 201 or the like defect being caused, i.e., a problem of being poor in the reliability in elongated period of usage.
Such a problem exists similarly even in case another resin is used in place of polyimide for an insulation resin layer constituting a plural wire layer portion so long as it is still deformed by compression similarly to polyimide by the force to which it is subjected at the time of the above described mounting. Further, it can be considered to use a glass epoxy resin or the like for a substrate in place of ceramic, but in this case the above problem exists entirely similarly.